"Intentional" was perhaps the wrong word for glibc, but BSD libc (at least on FreeBSD and OS X) actually document it as intentional: NOTES The fcl

"Intentional" was perhaps the wrong word for glibc, but BSD libc (at least on FreeBSD and OS X) actually document it as intentional: NOTES The fclose() function does not handle NULL arguments; they will result in a segmentation violation. This is intentional - it makes it easier to make sure programs written under FreeBSD are bug free. This behaviour is an implementation detail, and programs should not rely upon it. I thought I remembered similar wording for glibc, but I appear to have been mistaken.